The Point-to-Point protocol (PPP) is a network communication protocol specifically designed for connecting a device to a network, such as the Internet. In particular, a subscriber device, such as a home computer, a portable digital assistant (PDA), or other device, may utilize PPP to communicate with an Internet Service Provider (ISP) that provides access to the Internet or another network. PPP operates within the data link layer of the OSI model and, therefore, is referred to as a layer two protocol.
The Layer Two Tunneling Protocol (L2TP) defines a general-purpose mechanism for tunneling network packets conforming to PPP over various media. Specifically, L2TP defines a mechanism by which packets conforming to the PPP protocol are encapsulated within L2TP frames.
In a typical configuration, the subscriber device communicates with an L2TP Access Concentrator (LAC). The LAC is a device attached to the switched network access fabric, e.g., Public Switched Telephone Network (PSTN) or Digital Subscriber Line (DSL) network, or otherwise co-located with a PPP end system capable of handling the L2TP protocol. The LAC receives the PPP packets from the subscriber device, and passes the PPP packets to an L2TP Network Server (LNS) via an L2TP tunnel. The LNS, referred to generally as a “tunnel termination device,” is a layer three device that provides a termination point for the L2TP tunnel. More specifically, the LNS extracts the PPP packets encapsulated via the L2TP, and introduces the packets to the Internet or other network. Consequently, each L2TP tunnel is defined by an LNS-LAC pair in which the LNS and the LAC terminate opposite ends of the tunnel. Moreover, the LNS may be viewed as the “server-side” of the L2TP tunnel. Each L2TP session terminated by the LNS is referred to as a “subscriber session.” Each L2TP tunnel may include several subscriber sessions.
A typical ISP may have multiple LNSs to provide network access for subscriber devices. Upon receiving a network access request from one of the subscriber devices, the LAC typically selects one of the LNSs, and attempts to establish an L2TP tunnel with the selected one of the LNSs. If the attempt fails, the LAC selects a different one of the LNSs and repeats the process. This process continues until either an L2TP tunnel is established and a subscriber session has been established or all of the available LNSs have been tried.